Norovirus is one of four viral genera found in the non-enveloped positive strand RNA family Caliciviridae. The other three species in Caliciviridae are Lagovirus, Vesivirus, and Sapovirus. Sapovirus is the only member of the genus other than Norovirus which utilizes humans as hosts. The Norovirus genome is approximately 7.56 kb with three open reading frames (ORFs). The first ORF codes for nonstructural proteins including a helicase, a protease, and an RNA directed RNA polymerase (RDRP) all of which are required for replication of the virus. The remaining two ORFs code for Capsid proteins (Jiang, X. (1993) Virology 195(1):51-61). The numerous strains of Norovirus have been classified into 5 genogroups of which I, II, and IV infect humans (Zheng, D. P., et al. (2006) Virology 346(2):312-323) and are estimated by the CDC to cause approximately 23 million gastroenteritis cases, corresponding to 40% of foodborne illness each year in the US (Mead P. S. (1999) Emerg. Infect. Dis. 5(5):607-625).
Common symptoms are vomiting, diarrhea, and intestinal cramps. Vomiting is the most common symptom in children, while diarrhea is more common in infected adults. Dehydration is a significant concern. The loss of life due to this virus is about 300 patients per year in the United States, and these deaths are usually among patients with a weak immune system (Centers for Disease Control and Prevention. “Norwalk-like viruses:” public health consequences and outbreak management. MMWR 2001; 50 (No. RR-9):3). The incubation period from exposure to full infection is typically 24 to 48 hrs with approximately 30% of infected individuals showing no symptoms. Symptoms generally persist for 24 to 60 hrs (Adler, J. L. and Zickl, R., J. (1969) Infect. Dis. 119:668-673). Viral shedding may last for 2 weeks or longer following the infection, however, it is not clear whether this virus is infectious.
Norovirus is transmitted primarily by the fecal-oral route through contaminated food or water, person to person contact, aerosols of vomit or stool samples. Viral titers in stool samples can reach 106 to 107 particles per mL, and particles are stable to temperatures of 0° C. (32° F.) to 60° C. (140° F.) (Duizer, E. et al., (2004) Appl. Environ. Microbiol. 70(8); 4538-4543). The virus is highly infectious, and various sources suggest infection may require inoculation of as few as 10 to 100 viral particles (Centers for Disease Control and Prevention. “Norwalk-like viruses:” public health consequences and outbreak management. MMR 2001; 50(No. RR-9):3-6). This leads to epidemics in schools, nursing homes, cruise ships, hospitals, or other locations where people congregate.
Norovirus is named for Norwalk-like viruses, a name derived from an outbreak at a school in Norwalk, Ohio in 1968. The viral particle responsible for the Norwalk illness was identified in 1972 by immune electron microscopy following passage of rectal swab filtrates through three sets of human volunteers (Kapikian, A. Z. et al. (1972) J. Virol. 10:1075-1081). In following years, the virus was called small round structured virus due to its electron microscopic image, calicivirus since it a member of the Caliciviridae family, and/or probably most commonly Norwalk-like virus after the originally isolated strain. Common names for the virus include winter vomiting virus, stomach flu, food poisoning, and viral gastroenteritis. While the outcome of infection is generally non-life threatening, the cost of loss of use of facilities and loss of productivity is great, consequently, a therapy for treatment of Norovirus infection in humans would be very desirable.
There is currently no approved pharmaceutical treatment for Norovirus infection (http://www.cdc.gov/ncidod/dvrd/revb/gastro/norovirus-qa.htm), and this has probably at least in part been due to the lack of availability of a cell culture system. Recently, a replicon system has been developed for the original Norwalk G-I strain (Chang, K. O., et al. (2006) Virology 353:463-473). Both Norovirus replicons and Hepatitis C replicons require viral helicase, protease, and polymerase to be functional in order for replication of the replicon to occur. Most recently, an in vitro cell culture infectivity assay has been reported utilizing Norovirus genogroup I and II inoculums (Straub, T. M. et al. (2007) Emerg. Infect. Dis. 13(3):396-403). This assay is performed in a rotating-wall bioreactor utilizing small intestinal epithelial cells on microcarrier beads, and at least initially seems as though it would be difficult to screen a meaningful number of compounds with this system. Eventually the infectivity assay may be useful for screening entry inhibitors. Other groups, such as Ligocyte Pharmaceuticals, Inc. (http://www.ligocyte.com/) have focused on trying to develop a vaccine against Noroviruses, however, these efforts have not yet been successful and may prove difficult as has often been the case in viral systems where low replicase fidelity is an evolutionary benefit.
It would be advantageous to provide compounds, compositions and methods for treating Norovirus infections. The present invention provides such compounds, compositions and methods.